The present invention relates to an improved process for forming optical preforms for the production of optical fibers, this process being especially suited for making optical preforms which possess stepped index or graded index profiles. Such characteristics render it possible to draw optical fibers exhibiting reliable operating characteristics from such optical preforms.
There has been a continuous search in the prior art for economical and mass production of fiber optic cables for use in optical communications systems.
Thus, the prior art contemplates and describes techniques such as "soot" deposition or hydrolysis wherein a gas vapor mixture is hydrolyzed by a flame to form a glass precursor particulate. The particulate is then deposited on a rotating glass rod serving as a mandrel. The soot is deposited upon the mandrel in a perpendicular direction to provide successive layers of constant radius or to provide preforms with radial gradations by varying the dopant concentration in successive passes of the burner flame. The mandrel is then removed and the thus obtained cylindrical tubular preform is collapsed to a solid rod and then drawn into a fiber. This process is shown and discussed in U.S. Pat. No. 3,826,560 and U.S. Pat. No. 3,823,995. It is also known to use a plasma torch for producing the soot, and to deposit the soot on a boule at a temperature resulting in simultaneous sintering or fusing of the material of the boule, using a solid cross section plasma. This process is known as direct glass deposition. However, experience has shown that the optical properties of the material of the boule produced by the direct glass deposition method and of the optical fiber drawn therefrom are relatively poor, apparently owing to the very high temperature of the soot during its formation and travel toward the boule and the impossibility to control the composition of the boule at various regions thereof, which is caused by random deposition of the soot particles on the boule.
Other techniques as in U.S. Pat. No. 3,614,197 describe processes for continuously forming a fiber optic cable by using a multi-stepped funnel-shaped vessel to form a solid glass rod which is then heated and drawn into a fiber.
In any event, there is a desire to provide a solid optical preform and then draw or process the same into an optical fiber. Both the soot deposition process and the continuous forming approach have inherent benefits in the mass production of such cables.
U.S. Pat. No. 3,966,446 discusses a technique for providing an optical preform. The optical preform is fabricated by the axial deposition from a direction along the preform axis as opposed to radial deposition from a direction perpendicular to the preform axis. The technique does not require a mandrel and thus avoids the need for collapsing a cylindrical preform prior to drawing.
The preforms thus provided in the above noted patent possess longitudinal gradations in the index of refraction and thus serve to enhance certain types of mode conversions.
In any event, there is a need to provide large optical preforms which then can be drawn into elongated optical fiber cables. There is a further need to provide an optical preform which can exhibit stepped single mode or graded index profiles to enable the resultant cable to be used to more efficiently transmit optical information in the form of digital or other signals.
It is known that fiber cables which possess a single mode of operation alleviate mode dispersion problems. It has been a problem to produce reliable cables employing single mode operation in that the prior art techniques could not adequately control the composition of the cable. Thus, many cables employ a multi-mode operation in using radial gradations in the index of refraction. In these cables the difference in velocity from mode to mode compensates for the different path lengths and results in a relatively equal traversal time for all modes.
It is clear that in order to efficiently employ a single mode or a multi-mode operation, one must carefully and accurately control the fabrication of the fiber to assure that the same is consistent in formation and hence, possesses repeatable and reliable operating characteristics.
The fabrication techniques of all optical fiber preforms are broadly based on one fundamental principle, i.e. vapor phase deposition. For example, the reported processes are chemical vapor deposition (CVD), modified chemical vapor deposition (MCVD), outside vapor phase oxidation (OVPO), inside vapor phase oxidation (IVPO), vapor phase axial deposition (VAD) and plasma-activated chemical vapor deposition (PCVD). In all these processes, halides of the preform-forming materials are converted at high temperature to the respective oxide particles and the chemical conversion and deposition processes occur simultaneously.
As advantageous as the conventional application of these methods may be for the production of optical preforms of certain characteristic optical properties, they are either too expensive, too time-consuming or too cumbersome to use commercially, or they are not suited for the production of optical preforms having differentiated optical characteristic properties, and especially for the production of radially graded or stepped index optical preforms.